Method of preparing indirectly heated cathodes



Nov. 9, 1965 K. N. K. WILLMOTT ETAL 3,216,856

METHOD OF PREPARING INDIRECTLY HEATED CATHODES Filed May 1'. 1961 2 Sheets-Sheet 1 BLOCK CATHODE I SPRAY GUN 4 6 T COATING MATERIAL //Vl E/\/7 OF?5 KENNETH /V. K. W/LLMOTT MHLCOLM wLL W M ,4 T TOP/V5 Y Nov. 9, 1965 K. N. K. WILLMOTT ETAL 3,216,856

METHOD OF PREPARING INDIRECTLY HEATED CATHODES Filed May 1, 1961 2 Sheets-Sheet 2 T T W F|G.3.

//V VI N TOPS ATTQRA/EY I6 KENNETH MK W/LLMOTT United States Patent 3,216,856 METHOD OF PREPARING INDIRECTLY HEATED CATHODES Kenneth Nash Knight Willmott, Cheshunt, and Malcolm Reginald Farrell, Enfield, England, assign'ors to Associated Electrical Industries Limited, London, England, a British company Filed May 1, 1961, Ser. No. 106,593 Claims priority, application Great Britain, May 3, 1960,

15,588/ 60 3 Claims. (Cl. 117-222) This invention relates to indirectly heated cathodes for thermionic valves.

The invention is applicable both to diodes and to valves having grids and is especially applicable to valves which will in use be subjected to high voltages between heater and cathode, such as efficiency diodes.

In such valves the cathode, as is well known, comprises a metal sheath or casing, usually of rectangular or circular cross section, within which is a heater wire. In order to stand up to the voltage differences between heater and cathode sheath it is necessary to provide adequate insulation which must also be refractory and this must be of such a nature as to permit of easy assembly of the cathode without damage to the insulation, especially in the case of cathodes of very small dimensions such as are employed in so-called miniature and sub-miniature valves. Cathodes for such valves have hitherto usually been constructed 'of an insulated wire inserted within the metal shell. This however, gives only a limited thiclmess of insulation and since the heater must usually be coiled or looped in order to obtain the necessary length of wire, there is a risk of the coating being broken off at the bends during insertion, it being appreciated that the coating, being refractory, is liable to be brittle.

The main object of the invention is to provide an improved forrn of manufacture which avoids or minimises the above disadvantage.

The present invention comprises a method of manufacturing an indirectly heated cathode for a thermionic valve consisting in coating the internal surface of the cathode sheath with refractory and electrically insulating material by blowing a vapour suspension of said coating material through the interior of the cathode sheath, heating the sheath to stove the coating and thereafter inserting the heater.

Te coating material is preferably alumina with a binder in a vapourisable liquid suspension.

The binder may be silica seal, i.e. ethyl silicate, and the vapourisable liquid may be methylated spirits or acetone or a mixture thereof.

Alternatively to alumina, magnesium oxide may be employed.

Preferably the vapour suspension is blown through the cathode sheath by an air flow which entrains the coating material through a nozzle.

The surface of the actual heater wire should also be coated with a refractory insulating material. In some cases such coating may be reduced in thickness.

The term vapour suspension as used in this specification and in the appended claims is to be interpreted as a suspension of solid and/ or liquid particles in a vapour. The term liquid suspension is likewise to be interpreted as a suspension of solid particles in a liquid.

In order that the invention may be more clearly understood reference will not be made to the accompanying drawings, in which:

FIG. 1 shows pictorially an example of apparatus suit able for carrying out the invention.

FIG. 2 is a detail sectional view to an enlarged scale of the nozzle, and

ice

FIG. 3 is a horizontal section on the line III-III of FIG. 2.

In FIG. 1 the reference 1 indicates the cathode which is held in position in a vertical slot in a block 2 by means of a permanent magnet 3. 4 is the spray gun which projects the coating material vertically upwards through the interior of the cathode 1. The spray gun is actuated from an air line 5 and sucks the coating material up by entrainment through a pipe 6. Above the cathode is an exhaust tube 7 up which a further jet of air is blown from a second air line 8.

FIGS. 2 and 3 show the nozzle in greater detail and this consists essentially of a casing 9 housing a duct 10 up which the coating material is sucked and which terminates in a nozzle 11 directed into the lower end of the cathode 1. The air from the pipe line is fed through a duct 12 and into an annular space 13 around the internal duct 10. This air is forced upwardly and in the form of an annular jet around the nozzle 11 into the cathode 1. The air jet flowing upward entrains the coating material through the nozzle 11 and projects it into the cathode 1 in the form of vapour suspension from which it is deposited 0n the internal wall of the cathode. The annular air space 13 terminates in an upper end cap 14 to which it is sealed by a rubber seal 15 and similarly at the lower end of the nozzle the duct 10 is secured to a lower end cap 16 and sealed thereto by an annular rubber seal 17 compressed by the lower end cap.

Preferably the coating mixture consists of a suspension of alumina and silica seal in a mixture of methlyated spirits and acetone. T-he alumina forms the actual coating and the silica seal serves to bond it, whilst the methylated spirit and acetone serve as a carrier which is quickly evaporated. After the coating operation the cathode would be stoved at about 500 C. to set the coating firmly.

To coat cathodes of different cross sections both duct 10 and nozzle 11 and the upper end cap 14 can easily be changed for others of the correct dimensions. In order to prevent excess suspension being passed into the cathodes the quantity of mixture should be limited to the minimum necessary or the air pressure can be removed after the minimum has been applied; this could be controlled by electronic means.

What we claim is:

1. A method of manufacturing a sheath for an indirectly heated cathode for a thermionic tube including the steps of providing an elongated cathode sheath, coating the internal surface of the sheath by spraying into one end of said sheath a vapour suspension of an electrically insulating and refractory material and a silica seal binder and simultaneously applying suction to the opposite end of the cathode sheath and subsequently heating the cathode sheath to dry the coating.

2. A method of manufacturing a sheath for an indirectly heated cathode for a thermionic tube including the steps of providing an elongated cathode sheath, coating the internal surface of the sheath by spraying into one end of said sheath a vapour suspension of at least one of the materials of the group consisting of aluminium oxide and magnesium oxide and a silica seal binder, simultaneously applying suction to the opposite end of the cathode sheath and thereafter heating the cathode sheath to dry the coating.

3. A method of manufacturing a sheath for an indirectly heated cathode for a thermionic tube including the steps of providing an elongated cathode sheath, coatipg the internal surface of the sheath by spraying into one end of said sheath a vapour suspension of at least one of the materials of the group consisting of aluminium oxide and magnesium oxide and a silica seal binder, simultaneously applying suction to the opposite end of the cathode sheath and thereafter heating the cathode sheath in a reducing atmosphere to dry the coating on the internal surface and remove impurities from the external surface of the cathode sheath.

References Cited by the Examiner UNITED STATES PATENTS 577,496 2/97 Wallwork et a1 118300 1,719,988 7/29 Myers 313-340 1,826,510 10/31 Driggs 313340 Bain 2925.13 Van Gessel 2925.13

Stewart 118300 Morey 117-96 Mayer 117230 Tierney 117-231 Davis 117-231 JOSEPH B. SPENCER, Primary Examiner.

10 ARTHUR GAUSS, RICHARD H. EANES, RICHARD D. NEVIUS, Examiners. 

1. A METHOD OF MANUFACTURING A SHEATH FOR AN INDIRECTLY HEATED CATHODE FOR A THERMIONIC TUBE INCLUDING THE STEPS OF PROVIDING AN ELONGATED CATHODE SHEATH, COATING THE INTERNAL SURFACE OF THE SHEATH BY SPRAYING INTO ONE END OF SAID SHEATH A VAPOR SUSPENSION OF AN ELECTRICALLY INSULATING AND REFRACTORY MATERIAL AND A SILICA SEAL BINDER AND SIMULTANEOUSLY APPLYING SUCTION TO THE OPPOSITE END OF THE CATHODE SHEATH AND SUBSEQUENTLY HEATING THE CATHODE SHEATH TO DRY THE COATING. 